The terminally differentiated heart muscle cell of the adult mammal has irreversibly lost the ability to replicate its DNA, undergo mitosis and divide. Little is known about the mechanisms responsible for this loss of proliferative activity during development. Previous studies have established that DNA biosynthesis and cell proliferation cease in heart muscle of the rat by the third week of postnatal development. Temporally correlated with this loss of synthetic activity is a greater than 95% loss in the activity of the putative replicative DNA polymerase (DNA polymerase alpha) and another "DNA Enzyme" thymidine kinase. The long term goals of this research project are to determine why heart muscle cells lose the ability to replicate their DNA and undergo mitosis during early development and why muscle cells of the adult are unable to regenerate following injury such as that caused by myocardial infarction. To determine how the availability and/or activity of DNA polymerase alpha is controlled during the terminal stage of differentiation, the enzyme will be purified and antibodies prepared against the enzyme protein will be used to measure its rate of synthesis and degradation. Studies using several prokaryotic and eukaryotic DNA polymerases including homologus DNA polymerase alpha, will measure template availability and 3'-OH initiation sites in nuclei and chromatin to determine if DNA synthesis ceases because the replicative DNA polymerase is lost or whether the replicative DNA polymerase is lost because DNA synthesis ceases, i.e., which is cause and which is effect. Additional studies will examine the involvement of DNA-affinity proteins in the restriction of DNA replication and the control of DNA polymerase alpha activity during cell differentiation.